The present invention relates to method and a system for assisting a driver of a vehicle during operation in order to avoid an undesired situation based on a current driving scenario.
The undesired situation may for example represent a departure from a desired future trajectory of the vehicle. In other words, the invention is applicable for so-called lane keeping of a vehicle during operation. A current driving scenario that represents an unintentional lane departure may be determined based on the vehicle's position, direction and/or orientation with respect to a traffic lane (or road edge). Further, there are systems known, which are configured for monitoring the course of the traffic lane ahead of the vehicle, such as by monitoring lane markings using a vision system. The lane keeping support system is preferably configured to provide such guiding force only in situations in which the supply of such guiding force to the steering device is deemed to be appropriate after analysis of all input data, such as the course of the traffic lane ahead of the vehicle, further vehicles on the road and a predicted driving behaviour of the vehicle.
The guiding force exerted onto the steering device is resistive if counteracting the force applied by the driver onto the steering device, or supportive if acting in the same direction as the force applied by the driver onto the steering device, thus for instance reducing the effect of e.g. frictional forces acting on the wheels and the like which are experienced by the driver as resistance when operating the steering device. The steering device is normally formed by a conventional steering wheel in the case of a vehicle. However, the invention is applicable to other steering devices, such as a joystick, a sliding nipple or any other suitable steering device for steering the vehicle. For instance, in the case that the steering device is a steering wheel, the guiding force will appear as a guiding torque exerted onto the steering wheel.
According to U.S. Pat. No. 6,640,923, it is known to combine a power-assisted steering system with a lane keeping function. A steering actuator may reduce the torque to be manually applied for steering the vehicle and thus may relieve the driver. In addition, in power-assisted steering systems, it is believed that the torque support and/or the transmission ratio of the steering system are speed dependent. At lower speeds, such as those typical for parking and unparking, very direct steering having high torque support may be used, while during more rapid travel, indirect steering having low torque support may be desirable.
In many countries/regions there are legal requirements limiting the allowable guiding force to be applied to the steering device. According to a known method, the guiding torque applied to the steering wheel is automatically limited to the allowed limit during an intervention. However, such automatic limitation may lead to that the intervention is unsuccessful since the intervention could not be carried out to the desired extent.
It is desirable to achieve a method for assisting a driver which creates conditions for a further improved safety during operation, especially in case there is a predefined limit for the amount of the guiding force.
According to an aspect of the present invention, a method includes:                predicting if a first guiding force to a vehicle steering device is desired in order to avoid said undesired situation, and if the first guiding force is desired:        predicting a total guiding force comprising the first guiding force, which would be applied to the steering device for avoiding the undesired situation,        comparing the predicted total guiding force with a limit value, and if the predicted total guiding force exceeds the limit value,        in advance deciding whether to apply said predicted total guiding force to the steering device for avoiding the undesired situation or not.        
Thus, the total guiding force comprising the first guiding force would be applied to the steering device during an intervention for avoiding the undesired situation. Preferably, the intervention is performed during a driver steering operation. Preferably, the guiding force applied is only supportive, i.e. it is limited to such an extent that the driver still has full authority to steer the vehicle. However, the system may be configured to take control of the vehicle and in the case of lane keeping, return it to a safe position in the original lane.
In this way, a decision is made in advance, i.e. before an intervention is started, whether the intervention can be carried out to an extent leading entirely to the desired result. It has been acknowledged that in somecases it might be better not to apply the first guiding force at all than having to interrupt the intervention.
In other words, starting an intervention may provide the driver with a false appearance of safety since the supportive guiding force applied may not be sufficient for entirely avoiding the unintentional situation.
The total guiding force may comprise solely the first guiding force. However, according to a preferable example, the method comprises the steps of determining a second guiding force based on at least one guiding force operation model, and determining the predicted total guiding force as a sum of the first guiding force and the second guiding force. Thus, said second guiding force may be determined in accordance with other steering aspects, such as providing a certain steering feel to the operator, see further below. This example creates conditions for continuously controlling the guiding force applied to the steering device based on the second guiding force and only during said operation to avoid the undesired situation control the guiding force also based on the first guiding force.
According to an example embodiment, the method comprises the step of deciding to not apply said predicted total guiding force if it exceeds the limit value. Instead, it may be decided in advance to apply a limited total guiding force. Especially, the first guiding force, which is determined for avoiding the undesired situation (such as lane departure) may be cancelled all together.
According to a further example embodiment, the method comprises the step of deciding to apply said predicted total guiding force only if it is estimated to be sufficient for entirely avoiding the undesired situation.
According to a further example embodiment, the method comprises the steps of irrespective of any prediction of the first guiding force:                continuously during operation determining the second guiding force,        continuously applying the total guiding force comprising the determined second guiding force to the steering device, and        only if the first guiding force is desired determining the predicted total guiding force as said sum of the first guiding force and the second guiding force.        
This embodiment creates conditions for using an Electrical Power Assisted Steering (EPAS) system, Especially, the method is applicable in steering systems where there is a mechanical connection between the steering device and the ground but where the inherent steering feel resulting from the mechanical connection during operation is eliminated or at least suppressed. Such a system is known, in which a guiding, force (the second guiding force above) is continuously determined during operation so that the driver experiences a desired feel in the steering device instead of the inherent steering feel resulting from the mechanical connection.
For example, the inherent mechanical friction in the actual steering arrangement depends on different operational conditions, such as manufacturing tolerances, wear, temperature, age etc. Thus, the mechanical friction in the mechanical connection is different for different individual vehicles and varies over time. Thus, this embodiment creates conditions for decoupling the hardware (mechanical connection) from the friction steering feel. In other words, the embodiment creates conditions for an application-independent (hardware-independent) friction steering feel.
According to a further development of the last-mentioned example embodiment, said at least one guiding force operation model comprises at least one desired steering characteristic parameter. The steering characteristic parameter can be a guiding force influencing operational parameter. Said at least one desired steering characteristic parameter is preferably formed by vehicle lateral acceleration.
It is specifically advantageous to combine a safety function, such as lane keeping, with this type of EPAS since a certain lateral acceleration requires a certain amount of guiding force which already is determined in the EPAS system.
It is also desirable to achieve a system for assisting a driver which creates conditions for a further improved safety during operation, especially in case there is a predefined limit for the amount of the guiding force.
A system according to an aspect of the present invention is proivded comprising a means for predicting if a first guiding force to a vehicle steering device is desired in order to avoid said undesired situation, a means for predicting a total guiding force comprising the first guiding force, which would be applied to the steering device for avoiding the undesired situation and for comparing the predicted total guiding force with a limit value in order to in advance deciding whether to apply said predicted total guiding force to the steering device for avoiding the undesired situation or not if the predicted total guiding force exceeds the limit value.
According to an example embodiment, the system comprises a means for determining a second guiding force based on at least one steering device guiding force operation model, and a means determining the predicted total guiding force as a sum of the first guiding force and the second guiding force. Thus, said second guiding force may be determined in accordance with other steering aspects, such as providing a certain steering feel to the operator.
According to a further example embodiment, said at least one guiding force operation model comprises at least one desired steering characteristic parameter. Thus, the model can be designed to represent a nominal vehicle. Preferably, said at least one desired steering characteristic parameter comprises at least one of damping of steering device movements, tire friction, self alignment of the steering device to a neutral position and friction in a mechanical connection between the steering device and the wheels.
According to a further example embodiment, the system comprises a mechanical connection between the steering, device and the wheels for a mechanical transmission of steering signals from the steering device to the wheels, a means for decoupling a driver steering feel from the influence of the mechanical connection and for providing the driver with a desired steering feel based on the determined total guiding force. Preferably, a delivered steering device guiding force is measured and compared with an estimated desired steering device guiding force, wherein the delivered steering device guiding force is adapted by use of a feedback controller to be substantially the same as the desired steering device guiding force through adapting the amount of said guiding force.
Further preferred embodiment and advantages thereof emerge from the description below, the figures and the claims.